Document DE 44 34 978 A1, which is incorporated by reference, describes an active sensor with a two-wire interface which has a test mode for capturing a critical air gap length between sensor and encoder, wherein it is possible to change over between the normal mode of operation and this test mode by altering the operating voltage by virtue of the sensor being actuated with a serial voltage bit pattern.
Document DE 102 03 483 A1, which is incorporated by reference, proposes a wheel speed sensor which can be operated in different modes or has different modes of data transmission, wherein it is possible to change over between these modes of operation by means of external actuation using an additional input.